linux-kernel - Re: [v1 0/5] allow to reserve memory for normal kexec kernel

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Date:   Tue, 9 Jul 2019 12:59:22 +0100
From:   James Morse <james.morse@....com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     Bhupesh Sharma <bhsharma@...hat.com>,
        James Morris <jmorris@...ei.org>,
        Sasha Levin <sashal@...nel.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        kexec mailing list <kexec@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Catalin Marinas <catalin.marinas@....com>, will@...nel.org,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [v1 0/5] allow to reserve memory for normal kexec kernel

Hi Pavel,

On 09/07/2019 11:55, Pavel Tatashin wrote:
> On Tue, Jul 9, 2019 at 6:36 AM Bhupesh Sharma <bhsharma@...hat.com> wrote:
>> On Tue, Jul 9, 2019 at 2:46 AM Pavel Tatashin <pasha.tatashin@...een.com> wrote:
>>> Currently, it is only allowed to reserve memory for crash kernel, because
>>> it is a requirement in order to be able to boot into crash kernel without
>>> touching memory of crashed kernel is to have memory reserved.
>>>
>>> The second benefit for having memory reserved for kexec kernel is
>>> that it does not require a relocation after segments are loaded into
>>> memory.
>>>
>>> If kexec functionality is used for a fast system update, with a minimal
>>> downtime, the relocation of kernel + initramfs might take a significant
>>> portion of reboot.
>>>
>>> In fact, on the machine that we are using, that has ARM64 processor
>>> it takes 0.35s to relocate during kexec, thus taking 52% of kernel reboot
>>> time:
>>>
>>> kernel shutdown 0.03s
>>> relocation      0.35s
>>> kernel startup  0.29s
>>>
>>> Image: 13M and initramfs is 24M. If initramfs increases, the relocation
>>> time increases proportionally.
>>>
>>> While, it is possible to add 'kexeckernel=' parameters support to other
>>> architectures by modifying reserve_crashkernel(), in this series this is
>>> done for arm64 only.

>>
>> This seems like an issue with time spent while doing sha256
>> verification while in purgatory.
>>
>> Can you please try the following two patches which enable D-cache in
>> purgatory before SHA verification and disable it before switching to
>> kernel:
>>
>> http://lists.infradead.org/pipermail/kexec/2017-May/018839.html
>> http://lists.infradead.org/pipermail/kexec/2017-May/018840.html
> 
> Hi Bhupesh,
> 
> The verification was taking 2.31s. This is why it is disabled via
> kexec's '-i' flag. Therefore 0.35s is only the relocation part where
> time is spent, and with my patches the time is completely gone.
> Actually, I am glad you showed these patches to me because I might
> pull them and enable verification for our needs.
> 
>>
>> Note that these were not accepted upstream but are included in several
>> distros in some form or the other :)
> 
> Enabling MMU and D-Cache for relocation  would essentially require the
> same changes in kernel. Could you please share exactly why these were
> not accepted upstream into kexec-tools?

Because '--no-checks' is a much simpler alternative.

More of the discussion:
https://lore.kernel.org/linux-arm-kernel/5599813d-f83c-d154-287a-c131c48292ca@arm.com/

While you can make purgatory a fully-fledged operating system, it doesn't really need to
do anything on arm64. Errata-workarounds alone are a reason not do start down this path.


Thanks,

James